Fibrosis is characterized by excessive deposition of scar tissue by fibroblasts and it is currently one of the largest groups of diseases for which there is no therapy. Fibrosis is responsible for morbidity and mortality associated with organ failure in a variety of chronic diseases affecting the lungs, heart, kidneys, liver and skin. It has been estimated that nearly 45% of all deaths in the developed world are caused by or related to fibrotic conditions which include: cardiovascular disease, pulmonary fibrosis, diabetic nephropathy and liver cirrhosis (Wynn et. al., 2004).
Fibrosis and, especially, idiopathic pulmonary fibrosis (IPF) is a disease that is receiving increasing attention. The pathogenesis of fibrosis has been relatively undefined and only recently the various cellular and molecular processes that contribute to this disease have been unveiled. The overall consensus is that fibrosis is a result of an imbalance in the immune and repair response following infection and/or tissue damage (reviewed by Lekkerkerker et al 2012). These responses are the result of an intricate interplay between various cell types such as epithelial cells, fibroblasts, macrophages, fibrocytes, smooth muscle cells and endothelial cells. An imbalance in the activity in one or more of these cell types is expected to contribute to fibrosis.
Macrophages are responsible for immune surveillance and tissue homeostasis. They are able to engulf pathogens using a broad repertoire of pathogen recognition receptors (PRRs) and destroy them via degradation within lysosomes. Within the process of tissue homeostasis, macrophages play an essential role in removing dead and dying cells and toxic materials. Furthermore, macrophages are crucial in the orchestration of the wound healing process. To perform these important functions, macrophages consist of different subpopulations which are strategically positioned throughout the body (Mantovani et al., 2004).
During an immune response, monocytes are recruited from the circulation in the tissues and differentiate into macrophages. Following tissue damage and/or infection, macrophages exhibit primarily a pro-inflammatory phenotype and secrete pro-inflammatory mediators such as TNFα and IL-1. These pro-inflammatory macrophages are often called classically activated macrophages or M1 macrophages. Various chronic inflammatory disease and autoimmune diseases, such as, for example, rheumatoid arthritis, are associated with activation of M1 macrophages (Murphy et al., 2003).
To prevent an exacerbated immune response and collateral damage to surrounding tissue the M1 macrophage response needs to be tightly controlled. Macrophages that play a role in wound healing have been designated as alternatively-activated macrophages or, otherwise, M2 macrophages. This subset of macrophages secretes anti-inflammatory mediators and is strongly associated with Th2 mediated inflammation and antagonizes M1 macrophages to regulate the immune response.
A major initiator of fibrosis is the persistence of exogenous and endogenous stimuli of pathogens or tissue injury (Meneghin et al., 2007). Both classically activated (M1) and alternatively-activated (M2) macrophages are involved in the process of fibrosis. Nevertheless, M2 macrophages are considered to be the predominant macrophage subtype contributing to fibrosis (Song et al., 2000; Murray et al., 2011; Wynn, 2004). Furthermore, alveolar macrophages isolated from IPF patients are predominantly of a M2 macrophage phenotype (Thannickal et al., 2004).
A key characteristic of many fibrotic diseases is abnormal or exaggerated deposition of extracellular matrix degradation (ECM) (Cox et al, 2011). M2 macrophages can directly affect fibrosis by the excretion of pro-fibrotic mediators, such as tissue inhibitors of metalloproteinases and thereby directly inhibiting ECM turnover (Duffield et al., 2005). M2 macrophages also produce fibronectin, a key component of the ECM and thus contribute, directly to the buildup of excessive ECM. Besides the direct effect of M2 macrophages on fibrosis, M2 macrophages also indirectly contribute to fibrosis through activation of other cell types such as T cells, fibroblasts, and endothelial cells and thereby aggravating fibrosis (Wynn, 2008).
A hallmark of M2 macrophages is the production of CCL18, also known as pulmonary activation-related chemokine (PARC), and it is highly expressed in alveolar macrophages of IPF patients (Prasse et al., 2006, 2007, 2009). Other markers of M2 macrophages have been also identified, among them CD206 and CD163 (Mantovani et al, 2004). Prasse et al. showed that CCL18 concentration within the serum of idiopathic pulmonary fibrosis (IPF) patients strongly correlates with severity of IPF and is a predictive value for mortality (Prasse and Probst et al., 2009). In addition, CCL18 production is strongly increased in the lungs of patients with pulmonary fibrosis and affects cells such as fibroblasts, functioning directly as a pro-fibrotic factor (Atamas et al., 2003). Given that CCL18 is predominantly produced by M2 macrophages, it is likely that a misbalance between M1 and M2 macrophages favoring the M2 macrophages is involved in fibrosis. Recent studies have shown that M1 macrophages can convert into M2 macrophages indicating a dynamic balance between both macrophage subtypes (Duffield et al. 2005). Therefore, interfering in the M1/M2 balance, in particular preventing the occurrence of the M2 phenotype, provides a strategy to intervene in the process of fibrosis.
Over the past few decades much effort has been put into the development of in vitro and in vivo models to unravel the molecular mechanisms regulating fibrotic processes. Employment of primary cells and, preferably, those from fibrosis patients will provide us with better insights in the molecular processes involved in fibrotic disease. It is, however, important to use these cells under physiological conditions and in a disease-relevant context. The study of macrophages in functional assays relevant for fibrosis in combination with functional genomics can give invaluable insight into possible molecular mechanisms contributing to fibrosis and identify novel genetic targets for treatment of fibrosis. Therefore, there is a clear need to understand molecular and cellular processes related to fibrosis and to provide new methods of identifying targets, novel targets, and compounds useful for treatment of fibrosis.